1. Field of the Invention
This invention relates to a method for forming a carbide layer or a diffusion layer on the surface of an article to be treated such as a ferrous-alloy article or the like and a bath agent to be used therefor.
2. Description of the Prior Art
The ferrous-alloy article, such as a die and tools made of steel, with a carbide layer or a diffusion layer formed thereon, has greatly improved hardness and wear resistance. The above has well been known and industrialized.
The inventors have already developed a superior method for forming a diffusion layer or a carbide layer of a Va-Group element of the Periodic Table or of chromium on the surface of the article to be treated, such as the die and tools. In this surface treatment method, the article to be treated is immersed into a molten salt bath consisting essentially of borax, an oxide of a surface layer-forming element (hereafter: SFE), such as the aforementioned Va-Group element or chromium, and a reducing agent, such as aluminum, calcium, silicon or the like, to form the aforementioned surface layer (Japanese patent publication No. 4054/1978). According to this method, the reducing agent reduces the SFE-oxide to the metal thereby to recover the treating bath.
However, problems or disadvantages yet remain to be obviated in the aforementioned method. For example, when the content of reducing agent is too small relative to the amount of the SFE-oxide, there is sometimes formed no layer on the surface of the treated article. To the contrary, when the content of the reducing agent is too much, a boride layer is sometimes formed on the article, resulting in the formation of no diffusion or carbide layer of the aforesaid Va-Group element or of chromium. There are also encountered the following disadvantages. Depending on the kind of reducing agent, the viscosity of the molten bath increases. As a result, it is sometimes difficult to immerse the article to be treated into the bath, or the substance of the bath adhering to the surface of the treated article is hard to remove, which makes the surface layer on the treated article rough. Also, high viscosities result in nonuniformity of the formed layer.
To obviate the above-mentioned problems or disadvantages, the inventors engaged in extensive investigation and a large number of practical experiments, and have now completed the present invention.